1. Field of the Invention
The present invention relates generally to fuel injection control apparatus for internal combustion engines. More particularly, the invention concerns a fuel injection control apparatus for an internal combustion engine, capable of improving a minimum fuel injection quantity.
2. Description of the Related Art
Internal combustion engines are equipped with a fuel injection control apparatus that computes an appropriate fuel injection quantity according to the particular operational state of the engine and drives a fuel injector for supplying a fuel. The fuel injector opens or closes a valve constituting the injector, by utilizing the magnetic force generated by a built-in coil energized with the electric current allowing the injector to open the valve and to retain this open state, and thus injects the amount of fuel that is appropriate for the particular opening duration of the valve. The quantity of fuel injected is determined primarily by a differential between the pressure of the fuel and the atmospheric pressure of the injector nozzle, and by the time during which the fuel is being injected with the valve maintained in the open state. To inject the appropriate quantity of fuel, therefore, there is a need to set up the appropriate valve-open state hold time according to the particular fuel pressure and to open/close the valve rapidly and accurately.
However, during the time period from completion of power distribution to the injector to actual closing of the valve, the closing operation thereof is retarded by factors such as a delay in current circuit response. Traditionally, therefore, it has been a common practice to set up the power distribution time for the injector with the above response delay taken into account (i.e., a correction value has been added as an ineffective pulse signal width beforehand to injection pulse data computations).
In an alternative known method, when the supply current is switched from a high current for opening the injector valve (hereinafter, this current is referred to as the valve-opening current), to a low current for retaining the open state of the valve (hereinafter, this current is referred to as the hold current), the valve-opening current is rapidly discharged to minimize the response delay of the current circuit. This method is described in JP-3562125, for example.
In other known methods, in order to increase the valve-opening force of the injector according to fuel pressure, when a peak of the valve-opening current is reached, the supply time of the peak current is set to be variable, and when the injector pulse signal width is short, the peak current hold time of the valve-opening current is reduced. Thus, when power distribution to the injector is terminated, the injector is controlled to the hold current to stabilize the response delay of the current circuit. These methods are described in JP-A-2003-65129 and JP-3768723, for example.